This invention relates to a nuclear fuel rod and more particularly to an inexpensive fuel rod, wherein a tag gas for monitoring a fuel rod failure is sealed in the inner spaces of a cladding tube which surround the nuclear fuel pellets placed therein.
A nuclear fuel rod generally comprises, as shown in FIG. 1, a cladding tube 1 made of, for example, stainless steel or Zircaloy, a plurality of nuclear fuel pellets 2 placed one atop another in the cladding tube and a pair of upper and lower plugs 3, 4. A plenum spring 6 for restricting the displacement of the nuclear fuel pellets 2 is set in a plenum section 5 which is formed above a pile of the nuclear fuel pellets 2, or, if occasion demands, both above and below said pile. An inert gas such as helium (He) or liquid metal such as sodium is sealed as heating medium in the plenum section 5 and a pellet gap lying between the cladding tube and fuel pellet pile 2.
A large number of fuel rods constructed as described above are specially arranged to form a fuel assembly. Further, a large number of said fuel assemblies constitute the core of a nuclear reactor. The cladding tube of the fuel rod is primarily intended (1) to suppress a chemical reaction between the fuel pellet 2 and a coolant or moderator and (2) to prevent highly radioactive fission products from being released into the coolant or moderator. Should, therefore, the cladding tube be locally melted or cracked, then there will be the possibility that the coolant or moderator will be contaminated by radioactivity, presenting difficulties in the operation and control of a nuclear reactor. If, therefore, the cladding tube is damaged, it will be necessary to detect the leakage of radioactive material in the early stage and take proper countermeasures.
Hitherto, a defective fuel rod has been distinguished from among the numerous fuel rods constituting the core by means of the gas tagging method. This method comprises the steps of sealing different tag gases in the cladding tubes of the respective fuel rods; and, if it is found that the cladding tube of any of said fuel rods is damaged, analyzing a sample of a gas leaking, for example, into a coolant; and distinguishing the damaged fuel rod from the analyzed gas composition. The tag gas used for this object should be inert to a reactor material, nuclear fuel material, coolant and moderator. A tag gas used to date is formed of a gas mixture consisting of isotopes of noble gases such as Neon (Ne), Xenon (Xe) or Krypton (Kr) blended in various ratios.
A known process of sealing a tag gas in the cladding tube of a fuel rod includes the contactless mechanical type which comprises the steps of placing a small metal capsule containing a tag gas in the cladding tube and breaking up the metal capsule by electromagnetically moving a penetrator provided with a sharp end to release the tag gas ("Nuclear Technology", Vol. 26, pp. 472-479, 1975) and another type which comprises the steps of sealing a tag gas in a vessel made of an alloy having a lower melting point than the operating temperature of a nuclear reactor, inserting said vessel into a capsule, placing said capsule in the cladding tube of a fuel rod, and melting away the alloy by the high operating temperature of the nuclear reactor to release the tag gas (the Japanese patent publication No. 53-22635, 1978).
With the tag gas method, different forms of tag gas are generally used for a large number of fuel assemblies constituting a nuclear reactor. With a nuclear reactor containing 200 to 300 fuel assemblies, it is necessary to apply 200 to 300 different forms of tag gas. Since, one fuel assembly is formed of 50 to 200 fuel rods, and 50 to 200 capsules have to be used for one form of tag gas representing one fuel assembly, a total number of capsules used amounts to scores of thousands, thus consuming a tremendous amount of money for the manufacture of such numerous capsules. This is one of the reasons why the gas tagging method has now ceased to be used with a nuclear power reactor.